Switch to new public API endpoint, and deprecate partner subdomain

Parent task = Switch scholarphi to new public API endpoint #32158
- As seen in comment/question, we have 2 goals:
- GOAL 1 = use the latest graph/v1/ endpoint instead of legacy v1/ endpoint
- GOAL 2 = use the regular api.semanticscholar.org subdomain, instead of an optional partner.semanticscholar.org subdomain.
  - Question: before, users with S2_PARTNER_API_TOKEN env were calling the partner domain for higher rate limit. Can we use the same token on the regular api.semantic.org subdomain?
  - Rodney says yes.
THIS PR is for the main branch.
- Goal 1 was done in the chi-2021-demo branch as seen in this PR: #377
- So we'll be using that as a reference, MINUS the changes made for adding corpus IDs of citations.
- Goal 2 was done in the chi-2021-demo branch as seen in this PR: #382
- So we used that as a reference as well.

allenai / scholarphi